In a phased array ultrasound imaging system, an ultrasound transducer includes an array of transducer elements. The system includes a multiple channel transmitter and a multiple channel receiver connected through a transmit/receive switch to the transducer. Each transmitter channel causes a selected transducer array element to transmit an ultrasound pulse into an object being imaged, typically the human body. The transmitted ultrasound energy is steered along a transmit scan line and is focused by applying appropriate delays to the pulses transmitted from each transducer array element, so that the transmitted energy adds constructively a desired focal point to form a transmit beam. A part of the transmitted ultrasound energy is reflected back to the transducer array by various structures and tissues in the body.
Steering and focusing of the received ultrasound energy are effected in a reverse manner. The reflected ultrasound energy from an object or structure arrives at the array elements at different times. The received signals are amplified and are delayed in separate receiver channels and then are summed in a receive beamformer to form a receive beam. The delay for each channel is selected such that the receive beam is steered at a desired angle and is focused at a desired depth. The delays may be varied dynamically so as to focus the beam at progressively increasing depths along a receive scan line as the ultrasound energy is received. Ultrasound energy is transmitted along multiple transmit scan lines in a desired scan pattern, such as a sector scan, and the received signals are processed as described above to produce an image of the region of interest.
In order to obtain the highest quality image, both the transmit beam and the receive beam should be focused at each point in the area being imaged. However, the time required to obtain an image in this manner would be prohibitive. In most prior art systems, the transmit beam is typically focused at a single focal depth, and the receive beam is dynamically focused only in azimuth, the direction perpendicular to the divisions between elements of the transducer array. For both transmit and receive beams, the elevation focus is typically established by an ultrasound lens mounted on the transducer. As a result, the transmit beam is out of focus at points displaced from the transmit focal point, and the receive beam is out of focus in the elevation direction, except at a fixed focal point. These factors cause portions of the image displaced from the focal points to be degraded in quality.
U.S. application Ser. No. 08/006,084, filed Jan. 19, 1993 and now U.S. Pat. No. 5,301,168 and assigned to the assignee of the present application, discloses an ultrasound transducer having rows and columns of transducer elements. Two or more transmit beams are transmitted at the same steering angle, but at different focal depths. The received signals in the region of each transmit focal point are "spliced" together to form a single receive line at each steering angle. The transmitted beams are focused both in elevation and in azimuth by energizing different apertures of the transducer at different focal depths. Since the spliced receive line is made up of received signals from regions where the transmitted beams are relatively focused, image quality is improved. However, unless special measures are taken, the frame rate of the ultrasound images (the rate at which complete images are generated) is decreased in proportion to the number of beams transmitted at each steering angle. For ultrasound images in cardiac applications, frame rates lower than about 30 frames per second are considered unacceptable.
One way to increase the rate of data acquisition in an ultrasound imaging system is by generating two or more different receive beams simultaneously. This may be accomplished by replicating the beamforming hardware for each additional receive beam. U.S. Pat. No. 4,622,634, issued Nov. 11, 1986 to Fidel, discloses an ultrasound imaging system wherein ultrasound vectors obtained simultaneously are sequentially entered into a scan converter. U.S. Pat. No. 4,926,872, issued May 22, 1990 to Brock-Fisher et al, discloses a color flow imaging system which utilizes parallel processing of color flow scan lines. Parallel processing of received ultrasound information is also disclosed in U.S. Pat. No. 4,644,795 issued Feb. 24, 1987 to Augustine; U.S. Pat. No. 4,790,320 issued Dec. 13, 1988 to Perten et al.; U.S. Pat. No. 5,027,821, issued Jul. 2, 1991 to Hirama et al.; U.S. Pat. No. 4,886,069 issued Dec. 12, 1989 to O'Donnell; U.S. Pat. No. 4,893,283 issued Jan. 9, 1990 to Pesque; U.S. Pat. No. 5,121,361 issued June 9, 1992 to Harrison, Jr., et al.; and U.S. Pat. No. 4,252,026 issued Feb. 24, 1981 to Robinson.